Your missing a few points i made.
If you get 5V from a supply, it goes to the circuit, and then there is a ground wire to complete the circuit and it should go back to the supply ground since the voltage difference to ground will be relative to the supply source and not some other ground.
On the shield part: Imagine it's raining out and the shield is keeping the rain from getting on the wires inside of it. So the rain on the
shield takes the water to some spot where it is collected and will not touch the internal wires. That spot can be an imaginary lake and all the shields from different cables go to that lake and then a big pipe takes all the water away to get rid of it. So the shield is protecting the
internal wires from picking up any noise ( signals which make no sense to your hearing ) and providing an easy path to ground to get rid of it. So shield ground has a very different use then a circuit ground.
I deal in pounds per hour and gpm so when it comes to circuits i could be all wet.
RICH